This invention relates to a belt device having an endless belt wound around a plurality of rollers, and an image forming apparatus (such as a copier, a facsimile or a printer) that uses the belt device.
Conventionally, an image forming apparatus uses an endless belt as a conveyor belt that conveys a recording medium or a transfer belt that bears a toner image. FIG. 17 shows an example of the conventional image forming apparatus using the endless belt. The image forming apparatus shown in FIG. 17 has a belt device including a drive roller 102, a tension roller 104 and an endless belt 101 wound around the drive roller 102 and the tension roller 104. Four transfer rollers 108 are provided between the drive roller 102 and the tension roller 104. On the upper sides of the transfer rollers 108 in FIG. 17, toner image forming portions 107 are disposed in opposition to the transfer rollers 108 via the endless belt 101. Each toner image forming portion 107 includes a photosensitive body 105, a charging device 106, an exposing device 109 and a developing device 111. The charging device 106 uniformly charges the surface of the photosensitive body 105. The exposing device 109 exposes the surface of the photosensitive body 105 to form a latent image. The developing device 111 causes the toner to adhere to the latent image on the photosensitive body 105. A voltage with polarity opposite to the electrical charge of the toner is applied to the transfer roller 108.
The recording medium P is supplied by a medium supply portion 117 one by one. An alignment portion 118 corrects the skew of the recording medium P or the like, and feeds the recording medium P to the endless belt 101. By the rotation of the drive roller 102, the endless belt 101 moves (circulates) as shown by an arrow I, and feeds the recording medium P. The toner image formed on each photosensitive roller 105 is transferred to the recording medium P due to the bias voltage of the transfer roller 108. The recording medium P is further fed to a fixing portion 119 disposed on the downstream side of the toner image forming portions 107, and heated and pressed so that the toner image is fixed to the recording medium P.
FIG. 18 is a sectional view taken along line XVIII-XVIII in FIG. 17. FIG. 19 is a perspective view of the belt device. In FIG. 18, the photosensitive body 5 is shown by a dashed line, and the charging device 106 (FIG. 17) is omitted. A cleaning blade 112 is provided in opposition to the photosensitive body 105, for removing the residual toner from the photosensitive body 105. Seals 114 and 115 are provided for sealing the gap between the cleaning blade 112 and an accommodating portion 116. However, the seals 114 and 115 can not completely prevent the leakage of the toner, and therefore the toner may leak out of the accommodating portion 116.
The above described drive roller 102 and the tension roller 104 (FIG. 19) are supported by a common belt frame 103, but there is a gap J between the belt frame 103 and the endless belt 101. If the toner leaks out of the accommodating portion 116, the toner may falls from the accommodating portion 116 and may enter into the inner region of the endless belt 101 through the gap J between the endless belt 101 and the belt frame 103 as shown by an arrow K. In such a case, the toner may directly adhere to the inner circumferential surface of the endless belt 101, or may indirectly adhere to the inner circumferential surface of the endless belt 101 via the transfer roller 108 having a voltage with polarity opposite to the toner. Such a toner may be caught between the endless belt 101 and the drive roller 102 (FIG. 19), and may decrease the friction force between the endless belt 101 and the drive roller 102. Thus, there may be a difference between the rotational speed of the photosensitive body 105 and the moving speed of the endless belt 101. As a result, the transferring positions on the recording medium P at the toner image forming portions 107 may shift from each other, and therefore the image quality may be degraded.
In order to solve this problem, Japanese Laid-open patent publication HEI 9-258568 (Page 2, FIG. 1) proposes an arrangement having a cleaning blade mounted on the belt frame, which scraps the toner from the inner circumferential surface of the endless belt.
However, even if the cleaning blade is mounted on the belt frame, the scraped toner remains in the inner region surrounded by the endless roller. Therefore, the scraped toner may again adhere to the inner circumferential surface of the endless belt, and may be caught between the endless belt and the drive roller. As a result, the degradation of the image quality can not be prevented.